It is a conventional practice to form protective coatings on metal surfaces, such as surfaces of aluminum foils, sheets and plates, to protect them. In recent years, increasing size and weight reductions are under way in electronic equipment and communication equipment, leading to a stronger demand for size and weight reductions on secondary batteries to be employed as drive power supplies in them. To meet these demands, batteries represented by lithium ion secondary batteries having high energy density and high voltage have been proposed as replacements for conventional alkaline batteries.
Concerning electrode plates which considerably affect the performance of secondary batteries, on the other hand, it has been proposed to form them into thinner films of larger areas with a view to providing the secondary batteries with longer charge-discharge cycle life and higher energy density. As disclosed, for example, in Patent Document 1, Patent Document 2, etc., there have been disclosed positive electrode plates each of which is obtainable by adding a dispersion or solution of a conductive aid and a binder in a suitable moistening agent (solvent) to powder of a positive-electrode active material such as a metal oxide, sulfide, halide or the like to prepare a paste-like active material coating formulation, providing a collector made of a metal foil as a substrate, and then applying the coating formulation onto the substrate to form a coating layer (active material layer).
Further, capacitors each of which makes use of an electric double layer formed at an interface between a polarizable electrode plate and an electrolyte are used as backup power supplies for memories, and their application to those requiring a large output such as power supplies for electric cars is now attracting attention. For a large output, it is required to achieve both a high capacitance and a low internal resistance. Electrode plates for these capacitors are generally manufactured by applying a coating formulation, which is a mixture of a binder, an active material, a conductivity-imparting agent and the like, onto collectors and then drying the coating formulation like the above-described negative electrode plates for batteries.
As the binder for use in the above-described coating formulation for the battery or capacitor electrode plate, a fluorinated resin such as a polyfluorinated vinylidene or silicone-acrylic copolymer is used, for example. Negative electrode plates (batteries) and polarizable electrode plates (capacitors) are each obtained by adding a solution of a binder in a suitable moistening agent (solvent) to an active material such as a carbonaceous material to prepare a paste-like coating formulation of the active material and then applying the coating formulation onto a collector. In the above-described coated electrode plates, the binder employed to prepare the active material coating formulation is required to be electrochemically stable to a nonaqueous electrolyte and to be free from dissolution into the electrolyte of the batteries or capacitors, to remain free from swelling by the electrolyte, and further to be soluble in some solvent to permit the coating.